Young guinea pig sudden death

Question
My 5 and a half month old guinea pig died last night. Three days before his death he was not his usual self. He stayed hunched in his little hut all day and wasn't as playful as he usually is. I wasn't so worried because he was eating and drinking just fine, still got excited when I gave him his fave treat. His bowel movements were normal too. The morning before he died he ate as well, but the next treat I gave him he did not eat and I tried throughout the day to feed him and he was not interested at all. Not even when I gave him his fave treat. He didn't eat any of his pellets and he wasn't drinking either. He was having hardly any bowel movements and when he did they were not normal at all. Very very tiny. I was really worried, but I had an appointment for the vet the next day. I thought he would be fine until then, but later on that night I ended up taking him into an emergency clinic. He was so weak all day, hardly moving and not caring about anything. His eyes almost looked like they were sunken in as well. One of his teeth were missing and the vet told me that she noticed his stomach was caved in. She did not know though how he died. What may have happened to my little guinea pig? He was so young. Is there anything I could have done? I feel like I let him down. He has a brother that lived in the same cage as him. I don't want the same thing to happen to him.

Answer
I had a rescue guinea pig die in a similar way, same symtpoms, in the morning when i realised what had happened (at the time i put it down to timidness of a new cage mate) i found he had maggots up his rear end, whether they were there before he died i do not know but i think i would have spotted them.

my adivce for his brother would be to clean out his cage/hutch often, allow him grass grazing time for a well balanced diet and if you are very worried then arranage a check up with the vet for him explaining what had happened to your other piggy.

I am sorry to hear about your piggy, nature is a cruel thing